Light guides are increasingly being used for decorative and functional lighting purposes in various locations, some of which require the light guide to emit light selectively (e.g., comparatively uniformly or in a particular direction) along its length. Such light guides are referred to hereinafter as side-light guides. Various mechanisms are known for enabling light that is injected into a guide from a light source at one end to be extracted selectively from the guide along its length to provide, effectively, a linear lighting device. It is recognized that the use of side-light guides in linear lighting devices offers certain advantages: these include, for example, the possibility of using a low voltage light source such as an LED light source, and of separating the light source from the area in which the lighting device is actually located.
One form of side-light guide is described in EP-A-0 594 089 (in the name of Minnesota Mining and Manufacturing Company). The guide has light-extraction elements spaced along its length to cause light that is being propagated along the guide from at least one end thereof to be emitted selectively through a light-emitting region in the wall of the guide. In that case, the light-extraction elements comprise notches in the light guide, each notch having at least one reflecting surface of optical quality i.e. a surface that diffusely scatters only a small amount (generally less than 20%) of the light incident upon it. EP-A-0 956 472 (in the name of Minnesota Mining and Manufacturing Company) and EP-A-1 153 240 (in the name of 3M Innovative Properties Company) both describe other light guides of that type: in the guide described in EP-A-0 956 472, two sets of notches centered along different longitudinal axes are provided with a view to controlling the lateral distribution of light emitted by the guide while, in the guide described in EP-A-1 153 240, the notches have different notch angles with a view to producing a predetermined pattern in the illumination provided by the guide. Generally, light guides of that type offer the advantage that they can be designed to emit light very efficiently along their length and in a preferred direction with a pre-selected distribution, making them suitable for use in lighting devices in a wide number of applications.
Other forms of side-light guides are described, for example, in WO 99/22173, WO 00/25159 and WO 01/51851. WO 99/22173 (in the name of 3M Innovative Properties Company) describes a light guide comprising a core surrounded by cladding, in which the inner surface of the cladding is formed with indentations that cause light to be extracted from the light guide. A light-reflecting member may be provided around a portion of the light guide to redirect some of the extracted light in a particular direction. WO 00/25159 (in the name of Minnesota Mining and Manufacturing Company) describes a light guide comprising a core surrounded by a two-part cladding layer, in which the cladding incorporates various reflective materials to enhance the extraction and uniformity of the light emitted by the guide along its length. WO 01/51851 (in the name of 3M Innovative Properties Company) describes a light guide having light extraction structures spaced along its length to cause light to be directed from the light guide towards a diffuse reflective layer that, in turn, directs the light back through the light guide so that it is finally emitted through the region of the guide in which the light extraction structures are located.
Side-light guides are available in both flexible and rigid forms, and in materials that will not break or shatter and have a comparatively long useful life. They are also capable of providing a high level of illumination in an energy-efficient manner. Lighting devices incorporating side-light guides have already been used in various locations. They can, for example, be used as cost effective, safe, versatile alternatives to fluorescent tubes in commercial, architectural, and consumer lighting applications but can also be used in situations in which fluorescent tubes are impractical and/or to achieve effects that cannot be obtained using conventional lighting devices. For example, there is increasing interest in the use of side-light guides in lighting devices in vehicles and it has already been proposed to use such devices on the outside of a vehicle, to outline features such as the rear window or the boot. More recently, manufacturers have become interested in using side-light guides in lighting devices in car interiors for both practical and aesthetic reasons (see, for example, DE-A-100 31 527 in the name of Volkswagen AG and DE-A-102 59 623 in the name of Hella KG Hueck & Co).
When a side-light guide is used in a lighting device, it is usually necessary to support the guide in some way, to retain it in the required position and ensure that light is emitted in the required direction. Known methods of supporting side-light guides include the use of brackets or clips (for example, as described in U.S. Pat. No. 6,763,172 of 3M Innovative Properties Company), and channels or rails (for example, as described in US 2004/0240829 also of 3M Innovative Properties Company, in WO 99/22174 of Minnesota Mining and Manufacturing Company, and in the above-mentioned WO 00/25159). In the particular case in which a light guide is used as a lighting device for a car interior, the above-mentioned DE-A-100 31 527 and DE-A-102 59 623 describe that it may be located in a gap in the interior lining of the vehicle.
WO 2008/022007 (3M Innovative Properties Company) describes various forms of side-light light guides and supports for facilitating the assembly of lighting devices. More especially, for light guides having defined light-emitting regions, WO 2008/022007 is concerned with facilitating the installation of a light guide securely and in a particular orientation in a support to form a lighting device, advantageously without substantial detriment to the light output of the guide through the light emitting region.